Method and apparatus for predictably switching diversity antennas on signal dropout

ABSTRACT

A low-cost diversity antenna switching system and method is realized by controlling bias voltages on PIN diodes. By increasing the reverse bias voltage impressed upon a PIN diode, an RF signal impressed upon the diode is increasingly attenuated. As the PIN diode is forward biased to conduct in the forward direction, RF attenuation decreases. Two or more PIN diodes are used to increasingly attenuate signals from one antenna as attenuation of signals from another antenna is gradually decreased. The progression of the bias voltages is accomplished using a microprocessor that monitors a received signal strength indicator (RSSI) signal from a radio receiver. The RSSI is used to control which of two antennas are coupled into the receiver by predicting a signal fade.

FIELD OF THE INVENTION

[0001] This invention relates to diversity antenna switching systems. Inparticular, this invention relates to a low cost, diversity antennaswitching system for use with high fidelity audio equipment.

BACKGROUND OF THE INVENTION

[0002] Diversity receiving systems and diversity antennas are well knownin the communications art. In general, diversity antenna systems areused to accommodate the RF signal fading and multi-path signalpropagation anomalies that are common at VHF and UHF frequencies. Signalfading and multi-path signals can adversely effect reception of a radiofrequency's signal and, by selectively choosing one of two or morespatially separated antennas that pick up a signal, fading andmulti-path effects can be reduced. Multiple antennas are routinely usedwith cellular telephones and automobile radios.

[0003] Multiple antennas are now also used with wireless microphonesystems, which include a remote, low-power portable transmitter, thesignals of which are received and demodulated by a receiver. As awireless microphone is moved about a room, multi-path signals canadversely affect the demodulated audio output from the receiver becausemulti-path signals will produce phase anomalies in the reception processthat sound like popping noise or may even cause complete audio loss.Accordingly, diversity antenna systems are now employed in such wirelessmicrophone systems to overcome the adverse effects of multi-path andsignal fading.

[0004] At least one problem with a diversity antenna system is switchingthe appropriate antenna to the receiver in such a way so as toseamlessly couple the receiver to the proper antenna. Prior art systemsexist for selecting one, or a combination of two or more antennas to becoupled to a radio receiver. Some of these prior art systems aredisclosed in U.S. Pat. No. 5,777,693 to Kishigami, et al. for a“diversity receiving apparatus for a mobile unit;” U.S. Pat. No.5,517,686 to Kennedy, et al. for a “diversity Receiver for FM StereoUtilizing a Pilot Tone Multiple For Phase Alignment of ReceivedSignals;” U.S. Pat. No. 5,548,836 to Taromaru for a “DiversityReceiver;” U.S. Pat. No. 5,465,411 to Koike for a “Diversity ReceiverWith Switching Noise Reduction;” U.S. Pat. No. 4,293,955 to Gehr, et al.for a “Diversity Reception System;” U.S. Pat. No. 5,742,896 to Bose, etal. for a “Diversity Reception With Selector Switching at Super AudibleRate;” U.S. Pat. No. 5,697,083 to Sano for a “Diversity Receiver;” andsee U.S. Pat. No. 5,603,107 to Gottfried, et al. for a “Switching SystemFor Diversity Antenna FM Receiver.”

[0005] When high fidelity audio reproduction is required in a small sizepackage and at the lowest possible cost, prior art diversity antennaswitching system are too complex, too large, or too expensive. Prior artlow cost, switching systems also suffer from audio switch noise spikesthat they produce in the receiver when they perform a hard instantaneousswitch over from one antenna to another. A low cost, compact, method,and apparatus for selecting one or more antennas in such a way thataudio fidelity reproduction is maximized would be an improvement overthe prior art.

SUMMARY OF THE INVENTION

[0006] Using PIN diodes as variable RF signal attenuators, in seriesbetween each of the antennas of a diversity antenna system and the radioreceiver input in a wireless microphone system, the strongest signalreceived from a transmitter can be seamlessly selected at the receiverwithout producing noise spikes caused by phase differences between theantennas. The PIN diode will conduct radio frequency energy when biasedin the forward direction. By gradually biasing a PIN diode to conduct ina forward direction, its attenuation of a RF signal can be graduallyincreased and decreased. In a diversity antenna system, a PIN diodeconnected in series between the antenna and the receiver input, can begradually forward biased, thereby gradually reducing the attenuation ofRF signals passing through the diode from the antenna to the input.Simultaneously, another PIN diode connected in series with anotherantenna and coupled to the receiver input, can be gradually reversedbiased so as to gradually attenuate signal from the other antenna.

[0007] By gradually modulating the bias current of PIN diodes, they canbe used to progressively attenuate and de-attenuate signals from two ormore diversity antennas that are coupled to a common summing node thatis coupled to the input of the radio receiver. Signals received by oneantenna can be seamlessly combined with signals from another antenna soas to avoid sudden phase shifts that can produce unacceptable audiooutput noise spikes.

[0008] PIN diodes are small, inexpensive and easily controlled tomodulate their RF attenuation level.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 shows a schematic diagram of the preferred embodiment of adiversity receiving apparatus for coupling the signals from at least oneof two antennas into the input of a receiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0010]FIG. 1 shows a schematic diagram of the preferred embodiment ofdiversity receiving apparatus for coupling radio frequency signals fromat least one antenna of a pair of such antennas into a radio receiverfor demodulation. The diversity receiving apparatus 100 is comprised tworadio antennas 102 and 104 for detecting signals radiated from atransmitter not shown. While the antennas 102 and 104 are only shownschematically in FIG. 1, for improving signal reception, the actualantennas are preferably spaced as far apart as practical.

[0011] The first antenna 102 is capacitively coupled 112 to the cathodeof a first PIN diode 106. The anode of the first PIN diode 106 isconnected to a summing node 109 that is capacitively coupled 130 to theinput of a radio frequency receiver 110 which demodulates radiofrequency signals picked up at antenna 102 and outputs an audiofrequency signal. The second antenna 104 is also capacitively coupled123 to the cathode of a second PIN diode 108, the anode of which is alsocapacitively coupled 130 to the receiver input 110. As shown in thefigure, the anodes of the PIN diodes 106 and 108 are coupled to a commonnode 109 which is considered a summing node for the signals passingthrough the PIN diodes 106 and 108.

[0012] The diodes 106 and 108 act as variable RF signal levelattenuators by gradually modulating bias voltages applied to thesediodes. When the PIN diodes are reversed biased with a dc voltageapplied to the cathodes and anodes the PIN diodes block the passage ofRF signals across the PIN junction. As the diodes become forward biased,their attenuation of the RF signal decreases as the forward bias currentincreases eventually decreasing to substantially zero dB of attenuationwhen the PIN diodes are fully forward biased.

[0013] Bias voltage control of the PIN diodes is accomplished usingreactive networks that are driven by voltages supplied by amicroprocessor. The first PIN diode 106 is coupled to an output 140 of amicro controller 150 through a first reactive network comprised of aninductive radio frequency choke 114 in series with aresistive-capacitive network comprised of resistors 116, 120 andcapacitor 118 which together form a low pass filter. The time constantof the first filter is empirically determined to switch the bias voltageapplied to the cathode of the first PIN diode 106 appropriately fast orslow to accommodate the fading of signals at the antenna 102. The firstPIN diode 106 is forward biased by a voltage 136 delivered to the anodeof the PIN diode 106 through a current limiting resistor 132 and an RFchoke 134 which are connected in series to the summing node 109 asshown. The second PIN diode 108 is controlled using a second reactivenetwork 122, 124, 128 and 126 the components of which can preferably bematched to the component of the aforementioned first reactive network.

[0014] By virtue of the d.c. voltages impressed upon the anodes of thePIN diodes 106 and 108 by the power supply voltage 136, the PIN diodescan be controlled to variably attenuate signals coupled from theantennas 102 and 104 to the summing node 109 and subsequently to thereceiver 110 by controlling the plurality of the biased voltage appliedto the PIN diodes' cathode terminals. The outputs 140 and 142 of themicroprocessor 150, which are normally binary-valued voltages of either0 or 5 volts, are used to forward bias either one or both of the PINdiodes when the output voltage at pins 140 and 142 are set to zerovolts.

[0015] In operation, the +5 voltage from either pin 140 and 142 willeventually charge capacitor 118 and 126 according to the time constantestablished by the values of resistors 120 116, 132, 124, and 128 aswell as the values of capacitors 118 and 126. As the capacitors 118 and126 charge to the output voltage from the microprocessor, the PIN diodes106 and 108 will eventually become reversed biased attenuating radiofrequency signals coupled through them to the summing node 109.

[0016] When the output of the microprocessor 150 at either pin 140 or142 goes to a zero volt level, capacitors 118 or 126 will eventuallydischarge through the resistor 120 or 128 into the micro controllergradually forward biasing the respective diode 106 or 108 to aconductive state. As the PIN diode begins to conduct, its attenuation ofRF signals decreases thereby de-attenuating signals received at theantennas coupled to the receiver 110.

[0017] Control of which of the two PIN diodes 106 and 108 to forwardbias or reverse bias by the microprocessor 150 is determined by areceived signal strength indicator signal 156, developed by an output ofthe receiver 110. The received signal strength indicator signal (RSSI)156 is coupled through a low-pass anti-aliasing filter 157 to an inputport 154 in the micro controller 150 into an analog to digital converter152 preferably an included function of the microprocessor 150. In thepreferred embodiment the RSSI is produced by a Philips SA626 FM IFsystem.

[0018] The RSSI is preferably a dc signal level the amplitude of whichprovides an indication of the relative signal strength of the RF signalcurrently being received 110 from the summing node 109. As the signalstrength delivered to summing node 109 from either antenna 102 or 104changes, the amplitude of the received signal strength indicator 156from the receiver 110 will also vary. When the RF signal strength at theantenna 102 decreases, the received signal strength level will decreaseproviding an indication to the micro controller 150 that the signalstrength received by the antenna 102 is beginning to fade or perhaps bedestructively interfered with by a multi-path signal.

[0019] The microprocessor 150 is appropriately programmed toperiodically sample the amplitude of the RSSI signal 156. Themicroprocessor calculates a running average level of the RSSI andmonitors continuously whether this averaged signal strength isincreasing or decreasing. Using empirically derived data, when the RSSIlevel decreases below some predetermined threshold signal level, themicroprocessor 150 determines that a signal fade from the antenna 102 or104 currently coupled to the summing node 109, is beginning. Upon thedetermination that a signal fade is in progress, the microprocessor 150outputs an appropriate signal to the output pins 140 and 142 so as tobegin forward biasing the PIN diode (106 or 108) of the other antenna soas to begin gradually decreasing the attenuation of that antenna therebyincreasing the level of signal delivered to the summing node 109 fromthat antenna. After some delay the microprocessor outputs a logic one ora +5 output voltage to charge the other capacitor (118 or 126) so as toincrease the reverse bias voltage on the PIN diode 108 or 106 graduallysuppressing the amplitude of signal it delivered from the first orpreviously selected antenna to the summing node 109.

[0020] The determination of when to begin de-attenuating signals fromone antenna and attenuate signals from another antenna is made by themicroprocessor by converting the analog received signal strengthindicator (RSSI) signal to a numerical value and arithmeticallycalculating a running average of this numerical value using a previouslycalculated average signal level value. Peak values of the average RSSIlevel are recorded in microprocessor 150 memory for use in calculating athreshold RSSI level that is used to determine when to begin switchingantennas using the PIN diodes. This RSSI threshold level is less thanthe peak of the average RSSI level by an amount that is inverselyproportional to the peak of the average RSSI level, and directlyproportional to the dynamic range of the RSSI signal. The constants ofproportionality are determined heuristically.

[0021] If the average RSSI level goes below the threshold level, themicroprocessor 150 will output signals to pins 140 and 142 tode-attenuate signals from one antenna and attenuate signals from anotherantenna. The microprocessor will simultaneously replace the peak valueof the average RSSI with the current average RSSI level and recalculatea new threshold level. The threshold level is also dynamically adjustedby changes in the peak value of the average RSSI.

[0022] When RF signal levels at the receiver are strong, the diversityantennas need to be switched to prevent multi-path signals fromdestructively combining and adversely affecting audio quality of thedemodulated signals. When RF signal levels at the receiver weaken, thediversity antennas should be switched to prevent the receiver from goinginto squelch. At very low signal levels, no antenna switch should bemade. Determining when to switch antennas using the PIN diodes isaccomplished using a running average of the RSSI and a historical peakrunning average RSSI value.

[0023] The received signal strength indicator (RSSI) is an output from areceiver and is proportional to the strength of the RF signal at theantenna receiving the signal. In the preferred embodiment, the RSSI canvary from zero volts to +5 volts D.C. The RSSI is input 154 to ananalog-to-digital converter (A/D) within the microprocessor 150 andconverted to an 8-bit binary word having decimal values from 0-255. Themicroprocessor 150 stores the first such value, which in the preferredembodiment is named both “A2D₁₃ BESTVALUE” and “A2D_AVERAGE”.

[0024] A running average of the digitized RSSI (after conversion to adigital form by the AID) is continuously calculated by continuouslyre-reading the digitized RSSI and adding the most-recently read RSSIvalue latest to the previously stored A2D₁₃ AVERAGE and dividing theirsum by 2. The resultant average RSSI is stored as A2D₁₃ AVERAGE.Whenever the A2D₁₃ AVERAGE exceeds the value stored in A2D₁₃ BESTVALUE,A2D₁₃ AVERAGE is copied into A2D₁₃ BESTVALUE as a new value for A2D₁₃BESTVALUE.

[0025] The A/D converter samples the RSSI, and its output is used tocalculate a new A2D₁₃ AVERAGE once every 0.5 milliseconds correspondingto the A/D sampling rate. This rate was chosen to provide an optimumrate of change of A2D₁₃ AVERAGE. In a real-world environment, as A2D₁₃AVERAGE declines below an empirically determined threshold value, themicroprocessor will begin the antenna switch-over. The threshold forswitching antennas is expressed by equation 1:

A2D₁₃ BESTVALUE−A2D₁₃ AVERAGE>(No₁₃ RF₁₃ LEVEL+SAT₁₃ LEVEL−A2D₁₃BESTVALUE)·X  (1)

[0026] Where:

[0027] “No₁₃ RF₁₃ LEVEL”=RSSI level from the FM detector with no RFsignal input to the receiver;

[0028] “SAT₁₃ LEVEL”=RSSI maximum value, i.e. stronger RF signal levelsto the receiver will not produce a greater RSSI value;

[0029] “X” is determined by equation 2:

(No₁₃ RF₁₃ LEVEL+SAT₁₃ LEVEL−A2D₁₃ BESTVALUE)·X=1/Y·(SAT₁₃ LEVEL−NO₁₃RF₁₃ LEVEL) When SAT₁₃ LEVEL=A2D₁₃ BESTVALUE  (2)

[0030] Where “Y” is a constant and is set to provide the appropriateswitching level resolution. For the preferred embodiment, Y=8; SAT₁₃LEVEL=5.3 v.d.c.; NO₁₃ RF₁₃ LEVEL=1.25 v.d.c.

[0031] Substituting these values into equation 2 and letting A2D₁₃BESTVALUE=SAT₁₃ LEVEL gives:

(1.25v)·X={fraction (1/8)}·(5.3v−1.25v)

[0032] from which X can be determined to be equal to 0.405.

[0033] Substituting the values of X into equation 1 gives:

A2D₁₃ BESTVALUE−A2D₁₃ AVERAGE>(1.25v+5.3v−A2D₁₃ BESTVALUE) ·0.405  (3)

[0034] Where the right side of equation 3 is the switching threshold andis dynamically adjusted by the current value of A2D₁₃ BESTVALUE.

[0035] In operation, as a signal fade begins, as indicated by thereceived signal strength indicator 156, the microprocessor can begin tocouple the other antenna to the summing node 109 and after some delaybegin decreasing the signal from the fading signal delivered to otherantenna so as to provide a nearly seamless transition from one antennato the other. Unlike prior art diversity antenna switching systems, themethod and apparatus disclosed herein does not produce the audio signalanomalies from the output of the receiver 110 associated with hardswitching of one antenna to another.

[0036] While the embodiment shown in FIG. 1 depicts the use of reactivenetworks to produce a gradual biased voltage change to the PIN diodes106 and 108 alternate embodiments of the invention would includedirectly coupling the cathodes of the PIN diodes 106 and 108 to adigital to analog converter that is coupled to the microprocessor 150outputs. In such an embodiment, the microprocessor could output adigital representation of a desired bias voltage for the cathodes of thePIN diodes and directly control, in real time, the bias voltage appliedto the cathodes (or anodes) of the PIN diodes. Such an embodiment wouldprovide more close control of the PIN diode biasing but at an increasedparts cost. Reactive networks provide a low cost physically compactmeans by which the biased voltages of the PIN diodes can be controlledusing the micro controller outputs directly.

[0037] Still other embodiments of the invention would include reversingthe polarity or orientation of the PIN diodes 106 and 108 from thatshown in FIG. 1. Stated alternately, the cathodes of the PIN diodescould be coupled to the summing node 109 and forward biased (by eitherthe reactive networks or the output of a D/A) if the summing node werecoupled to ground potential, i.e. zero volts. A +5-volt output voltagefrom the micro controller 150 to terminals 140 or 142 would therebyforward bias the PIN diodes decreasing their attenuation.

[0038] Instead of using PIN diodes, still other alternate embodiments ofthe invention would include the use of gallium arsenide field effecttransistors instead of PIN diodes 106 and 108. By appropriately biasinggallium arsenide field effect transistors, they also can function asfirst and second variable RF signal level attenuators. Still othervariable RF attenuators would include bipolar junction transistors whichby appropriate bias voltages applied to the base terminals thereof canbe employed to increase or decrease RF signal levels passing throughthem to the summing node 109.

[0039] The PIN diodes used in the preferred embodiment are, of course,two-terminal devices and as shown in the topology of FIG. 1 the devicesanode's are common. The PIN diode anodes can be considered the firstterminals of such diodes. The PIN diode cathodes are considered to bethe second terminal of the diodes.

[0040] One skilled in the art will recognize that the PIN diodes beginto conduct in the forward direction and become forward biased when thevoltage measured from the anode to the cathode is greater than zerovolts and that the forward bias current will increase as the forwardbias voltage increases. The time constants of the RC networks (132, 120,116, 118,128,124 and 126) affect the rates of which the biased conditionof the PIN diodes change. By increasing the time constant of the RCnetworks the transition time of switching received signal from oneantenna to the other is increased. By increasing these time constantsthe switching time of the system becomes more susceptible to thecomplete signal dropout from one or both of the antennas. Alternatively,by shortening the time constants excessively the PIN diodes will moreabruptly attenuate signal from one antenna and more abruptly couplesignal from the other antenna to the summing node. The appropriate timeconstants need to be empirically determined to accommodate signal faderate in the intended environment of operation of the apparatus depictedin FIG. 1.

[0041] An alternate embodiment of the invention is depicted in FIG. 2.In this embodiment 200, RF attenuating PIN diodes 206 and 208 are notconnected to a common summing node. A first antenna 202 is capacitivelycoupled 212 to a first PIN diode 206. A second antenna 204 iscapacitively coupled 223 to a second PIN diode 208. Bias voltagesapplied to both the anodes and the cathodes of the PIN diodes 206 and208 are obtained from the microprocessor 250 through output ports 240,242, 243 and 244 of the microprocessor 250. Each PIN diode 206 and 208is capacitively coupled, 231 and 230 respectively, to the input of aradio receiver 210.

[0042] Instead of connecting one terminal of each of the PIN diodes to asumming node, as depicted in FIG. 1, which is then connected to theinput of a radio receiver, in the embodiment shown in FIG. 2, the biasvoltages applied to the PIN diodes of FIG. 2 are generated by othercircuitry, namely the microprocessor 250. Still other embodiments wouldinclude generating bias voltages by other circuits, includingadditional, dedicated processors.

[0043] In the embodiment shown in FIG. 2, the voltages impressed uponboth anodes and cathodes of the PIN diodes are controlled by themicroprocessor. Such an implementation requires that there be asufficient number of outputs from the processor 250 and that thevoltages applied to the PIN diodes be adjusted in magnitude and polarityso as to be able to appropriately adjust the attenuation of thecorresponding device. By appropriately controlling the polarity of thebias voltages impressed upon the diodes as shown in FIG. 2, theorientation or polarity of the diodes can of course be reversed whileretaining the functionality of the circuit as a diversity antennaswitch.

[0044] As shown in FIG. 2, variable voltages are applied to the diodesat the nodes coupled to the antennas and fixed voltages are applied tothe diodes on the opposite side of the diodes' junctions. Anotheralternate embodiment would of course include applying variable voltagesto the nodes of the diodes opposite the antenna and fixed voltagesapplied to the nodes of the diodes coupled to the antennas.

[0045] Still other embodiments of the invention would include using morethan two antennas which would of course entail using additional PINdiodes and reactive networks to control the biased voltages thereof.Alternate embodiments of the invention would include three, four or moreantennas coupled to a summing node 109 the corresponding PIN diodes ofwhich could be controlled by a resitive-capacitive reactive networkscontrolled by individual outputs for the micro controller 150. Such analternate embodiment of more than two antennas might also be configuredto apply bias voltages using the technique depicted in FIG. 2, i.e. thatmultiple antennas do not necessarily need to share a common summingnode.

[0046] I have disclosed a low cost, physically compact diversity antennaswitching system and methodology which can be used to input to areceiver, signals from at least one of several antennas which isselected according to the level of a signal representative of a signalfade (RSSI). As a signal fade begins, another antenna can be seamlesslyselected to possibly preclude complete signal dropout and audio signaloutput loss by changing the bias voltage on PIN diodes that act asvariable RF signal level attenuators.

What is claimed is:
 1. A diversity receiving apparatus for coupling signals from at least one antenna of at least first and second antennas to a radio receiver, said diversity receiving apparatus comprised of: a) a first variable, RF signal level attenuator having an input coupled to receive RF signals from said first antenna and having an output coupled to a RF signal summing node; b) a second variable, RF signal level attenuator having an input coupled to receive RF signals from said second antenna and having an output coupled to said RF signal summing node; c) at least one variable RF signal level attenuator controller, coupled to at least said first variable attenuator so as to control attenuation levels of said RF signal level attenuators; whereby RF signals received at said first and second antennas and coupled to said RF summing node can be selectively coupled to said radio receiver by controlling at least said first variable RF signal level attenuator.
 2. The diversity receiving apparatus of claim 1 wherein said first and second variable RF signal level attenuators are PIN diodes.
 3. The diversity receiving apparatus of claim 1 wherein said first and second variable RF signal level attenuators are gallium arsenide field-effect transistors.
 4. The diversity receiving apparatus of claim 1 wherein said first and second variable RF signal level attenuators are bipolar junction transistors.
 5. The diversity receiving apparatus of claim 1 wherein said at least one variable RF signal level attenuator controller is comprised of a reactive network.
 6. The diversity receiving apparatus of claim 1 wherein said at least one variable RF signal level attenuator controller is an R-C circuit.
 7. The diversity receiving apparatus of claim 1 wherein said at least one variable RF signal level attenuator controller includes a microprocessor.
 8. The diversity receiving apparatus of claim 1 further including an RF signal level attenuator controller coupled to said second attenuator.
 9. The diversity receiving apparatus of claim 1 wherein said control voltage source is a microprocessor providing an analog output voltage.
 10. The diversity receiving apparatus of claim 1 wherein said control voltage source includes a microprocessor that monitors historical signal levels from said first antenna, and, when said the average of said historical signal level from said first antenna drops below a predetermined threshold signal level, said microprocessor supplies a control voltage to said first and second reactive networks in order to substantially continuously increase attenuation of the currently received signal delivered to said summing node from said first antenna and to substantially continuously decrease attenuation of the currently received signal delivered to said summing node from said second antenna.
 11. The diversity receiving apparatus of claim 1 wherein said microprocessor includes a microprocessor that acts to de-attenuate signals from said second antenna prior to attenuating signals from said first antenna.
 12. The diversity receiving apparatus of claim 1 wherein said predetermined signal level of said signal from said first antenna is a relative signal strength indicator of radio signal strength generated by said radio receiver.
 13. The diversity receiving apparatus of claim 1 wherein said predetermined signal level of said signal from said first antenna is a signal indicating noise levels of audio signals demodulated from RF signals detected by said radio receiver.
 14. The diversity receiving apparatus of claim 1 wherein said RF signal summing node is coupled to a radio receiver input.
 15. A diversity receiving apparatus for coupling signals from at least one antenna of at least first and second antennas to a radio receiver, said receiving apparatus comprised of: a) a first PIN diode having a first terminal thereof coupled to receive RF signals from said first antenna and having a second terminal coupled to an RF signal summing node; b) a second PIN diode having a first terminal thereof coupled to receive RF signals from said second antenna and having a second terminal coupled to said RF signal summing node, said summing node being coupled to a D.C. voltage source; c) a first reactive network coupled to said first terminal of said first PIN diode to supply a variable voltage to said a first terminal of said first PIN diode; d) a second reactive network coupled to said first terminal of said second PIN diode to supply a variable voltage to said first terminal of said second PIN diode; e) a control voltage source coupled to said first and second reactive networks to supply a voltage to said first and second reactive networks; whereby signals received at said first and second antennas can be selectively coupled to said radio receiver by controlling at least said first variable attenuator.
 16. The diversity receiving apparatus of claim 15 wherein each said first terminal is an anode.
 17. The diversity receiving apparatus of claim 15 wherein each said first terminal is a cathode.
 18. The diversity receiving apparatus of claim 15 wherein said D.C. voltage source coupled to said summing node is a voltage source substantially equal to zero volts.
 19. The diversity receiving apparatus of claim 15 wherein said D.C. voltage source coupled to said summing node is a voltage source greater than zero volts.
 20. The diversity receiving apparatus of claim 15 wherein said D.C. voltage source coupled to said summing node is a voltage source less than zero volts.
 21. The diversity receiving apparatus of claim 15 wherein said first reactive network coupled to said first terminal of said first PIN is comprised of an R-C network having a predetermined time constant.
 22. The diversity receiving apparatus of claim 15 wherein said second reactive network coupled to said first terminal of said second PIN is comprised of an R-C network having a predetermined time constant.
 23. The diversity receiving apparatus of claim 15 wherein said control voltage source is comprised of a microprocessor.
 24. The diversity receiving apparatus of claim 15 wherein said control voltage source is a microprocessor providing an analog output voltage.
 25. The diversity receiving apparatus of claim 15 wherein said control voltage source includes a microprocessor that monitors historical signal levels from said first antenna, and, when said the average of said historical signal level from said first antenna drops below a predetermined threshold signal level, said microprocessor supplies a control voltage to said first and second reactive networks in order to substantially continuously increase attenuation of the currently received signal delivered to said summing node from said first antenna and to substantially continuously decrease attenuation of the currently received signal delivered to said summing node from said second antenna.
 26. The diversity receiving apparatus of claim 15 wherein said microprocessor includes a microprocessor that de-attenuates signals from said second antenna prior to attenuating signals from said first antenna.
 27. The diversity receiving apparatus of claim 15 wherein said predetermined signal level of said signal from said first antenna is a relative signal strength indicator of radio signal strength generated by said radio receiver.
 28. The diversity receiving apparatus of claim 15 wherein said predetermined signal level of said signal from said first antenna is a signal indicating noise levels of audio signals demodulated from RF signals detected by said radio receiver.
 29. The diversity receiving apparatus of claim 15 wherein said RF signal summing node comprises a radio receiver input.
 30. A diversity receiving apparatus for coupling signals from at least one antenna of at least first and second antennas to a radio receiver, said diversity receiving apparatus comprised of: a) a first variable, RF signal level attenuator having an input for coupling RF signals from said first antenna to an output, for coupling said RF signals to a radio receiver input; b) a second variable, RF signal level attenuator having an input for coupling RF signals from said second antenna to an output, for coupling said RF signals to a radio receiver input; c) at least one variable RF signal level attenuator controller, coupled to at least said first variable attenuator so as to control attenuation levels of said RF signal level attenuators; whereby RF signals received by at least one of said first and second antennas can be selectively coupled to said radio receiver input by controlling at least said first variable RF signal level attenuator.
 31. The diversity receiving apparatus of claim 30 wherein said first and second variable RF signal level attenuators are PIN diodes.
 32. The diversity receiving apparatus of claim 30 wherein said first and second variable RF signal level attenuators are gallium arsenide field-effect transistors.
 33. The diversity receiving apparatus of claim 30 wherein said first and second variable RF signal level attenuators are bipolar junction transistors.
 34. The diversity receiving apparatus of claim 30 wherein said at least one variable RF signal level attenuator controller is comprised of a reactive network.
 35. The diversity receiving apparatus of claim 30 wherein said at least one variable RF signal level attenuator controller is an R-C circuit.
 36. The diversity receiving apparatus of claim 30 wherein said at least one variable RF signal level attenuator controller includes a microprocessor.
 37. The diversity receiving apparatus of claim 30 further including an RF signal level attenuator controller coupled to said second attenuator.
 38. The diversity receiving apparatus of claim 30 wherein said control voltage source is a microprocessor providing an analog output voltage.
 39. The diversity receiving apparatus of claim 30 wherein said control voltage source includes a microprocessor that monitors historical signal levels from said first antenna, and, when said the average of said historical signal level from said first antenna drops below a predetermined threshold signal level, said microprocessor supplies a control voltage to said first and second reactive networks in order to substantially continuously increase attenuation of the currently received signal delivered to said summing node from said first antenna and to substantially continuously decrease attenuation of the currently received signal delivered to said summing node from said second antenna.
 40. The diversity receiving apparatus of claim 30 wherein said microprocessor includes a microprocessor that de-attenuates signals from said second antenna prior to attenuating signals from said first antenna.
 41. The diversity receiving apparatus of claim 30 wherein said predetermined signal level of said signal from said first antenna is a relative signal strength indicator of radio signal strength generated by said radio receiver.
 42. The diversity receiving apparatus of claim 30 wherein said predetermined signal level of said signal from said first antenna is a signal indicating noise levels of audio signals demodulated from RF signals detected by said radio receiver.
 43. A method of selectively coupling at least one of at least two antennas to a radio receiver comprising the steps of: a) obtaining a first sample of the relative radio frequency signal strength received by said radio receiver from a first antenna; b) calculating a running average signal strength level received by said radio receiver from said first antenna; c) calculating a radio frequency signal strength threshold signal level, below which signals from said first antenna are to be gradually attenuated prior to being coupled into said radio receiver while signals from said second antenna are to be gradually de-attenuated prior to being coupled into said receiver; d) when the radio frequency signal strength received from said first antenna drops below said threshold signal level, increasing the signal level input to said radio receiver from said second antenna and decreasing the signal level input to said radio receiver from said first antenna.
 44. The method of claim 41 wherein said step of obtaining a first sample of the relative radio frequency signal strength includes the step of: reading a received signal strength indicator.
 45. The method of claim 41 wherein said step of calculating an average signal strength level includes the step of: filtering said first sample of the relative radio frequency signal strength.
 46. The method of claim 41 wherein said step of calculating an average signal strength level includes the steps: a) converting said first sample to a numerical value; b) arithmetically calculating a running average of said numerical value.
 47. The method of claim 41 wherein said step of calculating a radio frequency signal strength threshold signal level includes the step of dynamically adjusting said threshold using a current value of said relative radio frequency signal. 